1. Field of the Invention
The present invention relates to scanning optical systems, and more particularly to a scanning optical system that not only corrects displacement of a position at which a beam of light scanning a surface to be scanned converges, but also has a light converging position detector for detecting the profile of the beam of light on the surface to be scanned.
2. Discussion of the Related Art
In scanning optical systems for recording images by scanning a recording medium, which is a surface to be scanned, with a beam of light, the light converging position (i.e., the beam waist position) must be located on the surface to be scanned with extremely high accuracy.
Further, scanning optical systems for use in laser printers and the like address the following problems. Even if the light beam converging position has been adjusted with high accuracy, changes in ambient temperature during the operation of the optical system not only cause optical parts constituting the scanning optical system and the surface plate carrying the optical parts to expand and contract, but also cause the indices of refraction of the optical parts to fluctuate, thereby displacing the light converging position from the surface to be scanned.
When the light converging position is displaced from the surface to be scanned, the diameter of a spot of the light beam irradiated onto the surface to be scanned is varied from a desired value, and as a result, the quality of the recorded image is impaired. Hence, the light source or the optical system must be moved in the direction of the optical axis so that the light beam converging position can be maintained on the surface to be scanned at all times even after the light converging position of the optical system has once been adjusted. To implement this, displacements of the light converging position from the surface to be scanned must be detected sequentially.
Disclosed as a solution to this problem are, e.g., Japanese Patent Unexamined Publication Nos. Sho 61-10769 and Hei 1-237614.
The publication No. Sho 61-10769 is designed to detect the profile of a spotlight by arranging a single light detector on a surface to be scanned and an image rotator for rotating the spotlight. The light detector is constituted by a knife edge.
The publication No. Hei 1-237614 is designed to detect a displacement of the light beam converging position on a surface to be scanned by arranging light detectors in front and at the back of the surface to be scanned, each light detector being constituted by a slit whose width is narrower than that of the light beam that is scanning the surface to be scanned.
However, the first conventional art is not designed to detect in which direction the light beam converging position is displaced when the light beam that is scanning the surface to be scanned becomes abnormal. Thus, the light beam converging position must be adjusted while the direction of the displacement is being traced, which makes the adjustment time-consuming.
Further, the second conventional art leaves the slits clogged by foreign matter such as dirt and dust since the slits are narrower than the diameter of the beam of light, which thus prevents the diameter of the spot of the light beam from being detected stably.
Still further, the second conventional art is not designed to detect an actual profile of the light beam on the surface to be scanned although it can detect in which direction, front or back, the light beam converging position is displaced and thus can effect such control as to cause the light converging position to near the surface to be scanned. However, there exists the problem that the actual profile of the light beam on the surface to be scanned cannot be detected.